In research and surgery of animals including small animals such as rats and mice, it can be extremely difficult to locate a terminus of a probe, electrode, micropipette, or other implement (herein generally termed a "probe") at a particular location within the subject's body without having to remove overlying structure and the like to permit direct observation of placement of the probe. This problem is especially critical in brain research involving the placement of a probe at a desired locus deep within a living subject's brain inside the surrounding skull.
To aid researchers in locating various anatomical structures in the brains of research animals such as mice, rats, cats, dogs, and primates, respective so-called brain atlases are often consulted. A brain atlas provides three-dimensional coordinates for the structures, normally using a Cartesian (rectangular) coordinate system, relative to one or more accessible anatomical features. (For example, for mice and rats, the usual reference feature on the skull is bregma, which is a point of meeting of the coronal and sagittal sutures. A second reference feature that is sometimes used in connection with bregma is lambda, which is located posteriorly of bregma and is a point of meeting of the lambdoidal and sagittal sutures. The sagittal suture connecting bregma and lambda is regarded generally as representing a sagittal mid-line of the skull.) However, despite the existence of such information, current apparatus and methods used to place an introduced probe are notoriously inaccurate with individual subjects and from one subject to another in a population of subjects. Such inaccuracy is a substantial problem because it results in unintentionally mis-positioned probes and other tools, which causes misleading research data and wasted animal resources.
Stereotaxic apparatus are known in the art for positioning a subject's head for brain research. For a small animal such as a mouse or rat, the head is held immobile by externally applied structures such as ear bars and a nose clamp providing a "three-point" holding system. As an example, reference is made to U.S. Pat. No. 5,601,570 to Altmann et al.
All known prior-art apparatus have various substantial shortcomings. For example, the Altmann et al. apparatus is inherently incapable of positioning a subject's head, in three-dimensional space, in a manner providing a high level of confidence that a probe inserted from outside the skull will "hit" a desired locus within the brain. More specifically, the Altmann et al. apparatus does not allow the researcher, intending to probe a living brain of a research animal, to position a particular animal's head in a manner providing reliably accurate insertion and placement of the probe to desired three-dimensional coordinates in the brain. The Altmann apparatus also exhibits poor precision of placements of a probe at a desired locus in each animal in a population of animals. Consequently, the researcher must conduct a series of "pilot" studies, followed by histological confirmations, to compare actual probe results with desired results (e.g., to compare actual hit loci with desired hit loci based on information in a brain atlas). Such studies using conventional apparatus usually produce data exhibiting wide variations that often are attributed wrongly to biological variations among individual animals in a population, strains, ages of animals, and so on. As the pilot studies progress, the coordinates provided by a conventional apparatus are adjusted gradually to compensate for the variation and to improve the hit rate. Unfortunately, such studies are time-consuming and costly to perform, and require substantially increased numbers of animals to conduct a particular experiment. Conventional instruments simply do not allow the researcher to differentiate between the many sources of error. Furthermore, even with adjustments to the apparatus based on the pilot studies, hit rates remain disappointingly low, resulting in inconclusive research.
As noted above, individual animals (even of the same strain) exhibit substantial variation, one animal to the next, in morphology of body structures such as the skull. If positioning of the body or body structure is guided, according to the prior art, solely on the basis of external features (e.g., positions of ear holes relative to each other and to the snout), this variation usually results in excessive variation in probe placement at target loci within the brain.
In view of the foregoing, there is a need for stereotaxic alignment systems, and for implements usable with such systems, configured so as to substantially improve (relative to conventional systems and implements) the accuracy with which loci on or in a body are "hit" using a tool or the like presented to the body.